1. Field of the Invention
The present invention relates to a forged product and a method of making such a product.
2. Description of the Related Art
An upper bracket, an under bracket and other supporting members are used as members for fixing the handlebars and front fork of a motorcycle. An example of upper and under brackets (which will be collectively referred to herein as “handlebar supporting members”) is shown in FIGS. 8 and 9.
As shown in FIGS. 8 and 9, each of the upper bracket 700 and under bracket 800 includes bosses 2a, 2b and 2c. The bosses 2a and 2b at both ends have through holes 1a and 1b, into which a front fork 30 is inserted. Although the front fork 30 shown in FIG. 9 is inserted into just one of the two through holes 1a and 1b, the front fork 30 is actually inserted into the other through hole 1b as well. Meanwhile, the boss 2c at the center has a through hole 1c, into which a steering shaft 40 is inserted. The rider can turn the front wheel to any direction he or she likes with the handlebars since the front fork 30 and the steering shaft 40 are supported by the upper bracket 700 and the under bracket 800.
To increase the fuel consumption and cut down the exhaust gas, a lot of people think it is important to reduce the overall weight of a motorcycle as much as possible. For that purpose, in the prior art, the upper bracket, under bracket and other handlebar supporting members are made of an aluminum alloy, which has a lighter specific gravity than steel. A handlebar supporting member made of an aluminum alloy is usually formed by subjecting a cast product, which has been formed by a casting process such as die casting, to a T6 heat treatment (including a solution treatment and an artificial aging treatment) in order to increase its mechanical strength.
Meanwhile, methods of making a magnesium alloy, which has an even lighter specific gravity than an aluminum alloy, have been developed. Also, techniques for increasing the strength and machinability of a magnesium alloy have been developed, too. That is why the handlebar supporting members are now preferably made of a magnesium alloy, not an aluminum alloy. The specific gravity of a magnesium alloy is approximately two-thirds of that of an aluminum alloy. For that reason, the weight of a motorcycle can be further reduced by using a magnesium alloy.
If a magnesium alloy is used, however, it is difficult to achieve a sufficient mechanical strength just by subjecting the cast product to a heat treatment. Thus, Japanese Patent Application Laid-Open Publication No. 6-172949 proposes that after a cast product made of a magnesium alloy has been forged, the forged product be subjected to a T6 heat treatment. On the other hand, Japanese Patent Application Laid-Open Publication No. 2002-254132 proposes that after a billet of a magnesium alloy that has been formed by a continuous casting process is subjected to a hot extrusion process, the billet be treated by a solution treatment, a forging process and then an aging treatment in this order. According to any of these techniques disclosed in Japanese Patent Application Laid-Open Publication Nos. 6-172949 and 2002-254132, the mechanical strength can be increased sufficiently by a plastic deformation to be caused by a forging process on the workpiece.
However, the present inventors discovered and confirmed via experiments that when the technique including the forging process as disclosed in Japanese Patent Application Laid-Open Publication Nos. 6-172949 and 2002-254132 was simply applied to making a handlebar supporting member, the workpiece would experience unexpected deformation (or would be bent) at an early stage of the forging process.
If such a deformation were produced in the workpiece, the workpiece would be deformed plastically so much that an increased press tonnage should be applied to get the forging process done and the die would have decreased durability as a result. Also, if such a deformation were produced, the workpiece might sometimes chip (that is, have a shape defect, e.g., an underfill) at the edges. Such chipping could be avoided by subjecting a workpiece that has been formed in a bigger size to a forging process. In that case, as the workpiece has a bigger size than required, an increased amount of burr will be left after the forging process, thus decreasing the yield (also referred to as the “material utilization factor”) of the material. On top of that, since a magnesium alloy is much more expensive than an aluminum alloy, such a decrease in material yield would increase the overall manufacturing cost significantly.
One reason why the workpiece has such a deformation is that the through hole of the handlebar supporting member has a portion to be formed by the forging process and another portion to be formed after the forging process. If the handlebar supporting member is made by a method including the forging process, the through hole of the handlebar supporting member is made by making a non-through hole once in a workpiece by the forging process and then removing a bottom portion of the non-through hole by a machining process, for example.